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Artikelaktionen

Control of London dispersion interactions in molecular chemistry

Lutz Ackermann, Göttingen


Control of London Dispersion Interactionsin Metal-Catalyzed C-H Activation

 

In this project, we will explore the use of attractive London dispersion energy interactions to influence activity and selectivity in metal-catalyzed C–H functionalization chemistry. Despite of considerable recent advances in C–H functionalizations, fundamental aspects of the key C–H activation continue to be poorly understood. Thus, a systematic study on attractive London dispersion energy interactions for catalyst formation and transition-state stabilization has unfortunately as of yet proven elusive. This program will address three key challenges, namely (i) the quantification of dispersion interactions for C–H activations, (ii) the development of dispersion-energy donors DEDs for transition-metal catalysis, and (iii) the rational design of catalysts for the diastereo-, meta- and enantio-selective activation of C–H bonds through dispersion interactions. The detailed understanding of attractive dispersion interactions by experimental and computational mechanistic studies will stimulate the development of DEDs for transition-metal catalysis in C–H functionalizations and beyond. Success will entirely rely on a tight interplay between preparative chemistry, spectroscopy and theoreticians. The project will thus yield strong interactions with other activities in the collaborative network.

 

Alexander A. Auer, Mülheim and Michael Mehring, Chemnitz

 

Heavy main group elements as dispersion energy donors - experimental and theoretical studies of bismuth compounds with bismuth-pi-interactions as structure determining component


In the framework of this project and based on our previous results we aim at achieving the following objectives: In order to assess and extend the general principles for heavy main group element···π interactions derived in the first funding period, we will systematically extend the molecular libraries of structurally related compounds. These novel compounds will be the basis for high-level electronic structure calculations in order to supplement the experimental results and rationalize the basic influences in complex structures In addition to crystal structure analysis and PXRD, spectroscopy should be used as a probe for weak molecular interactions with heavy main group elements. For this purpose, we will increase our efforts in the search for sensitive spectroscopic techniques that will complement the experimental and computational work. Here, techniques like NMR and Raman/IR spectroscopy will be explored in more detail. To further quantify the bonding energy for dispersion type interactions, dispersion-type driven control of polymorphism will be analysed. Furthermore, the dimeric, dispersion type driven structural motif of triorganobismuth compounds BiAr3 will be tested for its potential as building unit in supramolecular chemistry.

 

Raphael Johann Friedrich Berger, Salzburg and Norbert W. Mitzel, Bielefeld

 

Intramolecular dispersive interactions in the gas phase: experimental reference data and comparison with solid state and theory

 

Our project focusses on applying gas electron diffraction (GED) and other methods to explore the precise geometrical structure of a range of free molecules with intramolecular dispersion interactions. The results obtained for free gas-phase molecules will be compared with data obtained in the solid state by single crystal X-ray diffraction (XRD). Such studies in different phases will clarify whether and to what extent intermolecular solid-state effects change molecular structures as well as the occurrence and strengths of dispersion interactions. Most of the experimental data on larger, more complicated systems are derived from solid-state methods, but the fundamental interactions are preferably studied by gas-phase techniques. The question of comparability is thus obvious. There is also enormous progress in quantum-chemical (QC) method development for describing increasingly larger systems including a thorough treatment of dispersion. However, such calculations are usually applied to single molecules – again different from solid-state results. Comparison of experimental data for free molecules with a range of state-of-the-art QC calculations will help to evaluate the quality of such theory-approximations. With a set of data from gas-phase and solid-state methods as well as QC, we will be able to study method- or phase-dependence of dispersion interactions. In the first project phase we have demonstrated that our approach indeed provides valuable structural and thermochemical information on the occurrence and strengths of intramolecular dispersion interactions. The objects of study stemmed from own preparative work (e.g. interactions C6H5/C6F5, Cu···Cu or Hg···Hg) and from co-operations with other SPP groups (e.g. alkyl/alkyl in large diamantyl dimers). In some cases we found severe differences between gas-phase experiments and highest-level QC calculations, in others good agreement with some QC methods. We also established new ligand systems for synthesizing volatile dinuclear gold complexes. In this way we learned to generate a range of new molecules for studying certain types of dispersion-dominated interactions in isolated form. In the second funding period we will make use of this knowledge and deepen our understanding of intramolecular dispersion interactions, as well as produce challenging new objects plus experimental data as references for dispersion-corrected QC methods. We also aim at understanding the failure of certain methods for certain types of compounds. The various types of intramolecular interactions to be studied include a) σ∙∙∙ σ interactions in hydrocarbons and organosilanes, b) arene π∙∙∙π interactions, c) σ-hole interactions (halogen and chalcogen bonds), and d) d10∙∙∙d10 (e.g. Au∙∙∙Au, Hg∙∙∙Hg) and d10∙∙∙s2 interactions (e.g. Au∙∙∙Bi). We will also undertake GED structure determinations for at least four other groups in the SPP.

 

Albrecht Berkessel, Köln

 

Dispersion Interaction in Metal- and Organocatalysis - Assessment and Implementation in Salalen Ligands and N-Heterocyclic Carbenes

 

Understand - quantify - use: Our studies aim at the assessment and implementation of dispersion control in two areas of small-molecule homogeneous catalysis: (i) titanium-salalen complexes for the asymmetric epoxidation of non-functionalized and in particular terminal olefins with hydrogen peroxide, and (ii) N-heterocyclic carbenes for Umpolung reactions in organocatalysis and as ligands in metal-catalyzed C-H activation. By the introduction of dispersion energy donors (DEDs) into the two catalyst systems, significant experimental progress and mechanistic insight was achieved in both areas in the course of the first funding period. For the second funding period, we are intending to build on this body of information, and to provide, by rational approach, novel, readily available and broadly applicable catalyst generations. Mechanistic studies will aim at a thorough understanding of their modes of action, and in particular of the role of dispersion. This multi-level approach is intended to contribute, on one hand, to our understanding of dispersion control of molecular catalytic transformations. On the other hand, by rational incorporation of dispersion elements, we are aiming at novel catalyst generations of unprecedented activity and selectivity. The project will combine, in a synergistic manner, synthesis, spectroscopy, mechanistic analyses, and quantum-chemical computation. Several cooperations with other research groups are integral part of this endeavor.

 

Frank Biedermann, Karlsruhe

 

Experimental dissection of dispersion energies from electrostatic contributions and solvent effects in face-to-face pi-stacking complexes

 

We propose a novel experimental toolbox for evaluating the binding energies of face-to-face “stacking” aromatic systems, which will be of help to separate electrostatic from dispersive contributions for typical pi-pi complexes. Our approach is complementary to existing experimental approaches, e.g. studies with association complexes of small molecules, the molecular balance technique or the double mutant cycle analysis, which have to date not been fully conclusive in scrutinizing competing theoretical models about pi-pi stacking interactions. Stable and geometrically well-defined rotaxanated complexes of the large macrocycle cucurbit[8]uril and a suitable aromatic component will serve as the receptor moiety for binding a second aromatic guest. In this design, both aromatic compounds, i.e. the rotaxanated one, and the incoming second aromatic guest, are hold in a face-to-face orientation (=> high geometric control) and are each almost completely shielded from contact with solvent molecules (=> minimization of solvation effects on the overall pi-pi stacking energy). We also propose a path how the desolvation energy cost of the binding partners can be experimentally accounted for without the need for computational treatments. Both the high geometric control and the subtraction of solvation effects are important advantages over other association-complex-based studies. Furthermore, it will be straightforward to generate a data library, because the second aromatic binding partner can be readily mixed in and does not need to be covalently tethered, as is the case for molecular-balances. In fact, our supramolecular approach also avoids probing a “non-optimal” binding geometry, which can be a shortcoming for covalent molecular-balance setups. In the proposed line of research, we will first prepare the rotaxanated complexes and will then measure by isothermal titration calorimetry (ITC) the binding enthalpies and free enthalpies of their pi-pi-complex formation with aromatic second guests. Having direct access to binding enthalpies will facilitate the comparison to computed pi-pi interaction energies, as the computationally difficult to treat entropic component can be disregarded. Having access to free energies (and thus entropies) will provide a valuable data sets to test future improved theoretical models. Information about the pi-pi stacking geometry will be obtained by structure-based methods such as NMR spectroscopy in solution and X-ray diffraction structure analysis of crystallized complexes. By systematically varying both the aromatic binding partners, including systems with typical “polar” substituents or with “dispersion donors”, we will aim towards an experimental separation of electrostatic and dispersive effects for the face-to-face pi-pi interaction motif. This study will complement the emerging picture of the importance of dispersive interactions for molecular recognition and self-assembly.

 

Peter Chen, Zürich

 

Large Dispersion Effects in Organic and Organometallic Thermochemistry, Stereochemistry, and Reaction Mechanisms


This project investigates two structural classes of molecules, characterized in the first phase of SPP 1807, for which gas-phase bond energies and the corresponding equilibria in solution can be measured experimentally. With a wide range of substituents, the effect of dispersion on the bond strengths and equilibria can be quantified and compared to quantum chemical calculations with and without implicit solvation models. The results are general for all bond-making or bond-breaking processes, i.e. all chemical reactions, and they provide a rigorous test of the adequacy of present theoretical models.

 

Guido Clever and Ricardo Mata, Göttingen

 

Experimental and Computational Insights into Dispersion Interactions in Self-Assembled Supramolecular Host-Guest Systems

 

This project deals with the experimental and theoretical study of supramolecular host-guest systems, whereby the structural stability of the hosts and binding affinity towards guest molecules is strongly conditioned by dispersion interactions. Self-assembled coordination cages based on acridine/acridone functionalized backbones will be synthesized and characterized with a large variety of spectroscopic tools, including multidimensional NMR techniques, ion mobility mass spectrometry, X-ray crystallography and isothermal titration calorimetry. A modular synthetic approach will allow the construction of molecular cages of varying size and shape, as well as the introduction of dispersion energy donor groups in the binding pocket(s). Such structures are unique in character, as they include some of the complexity found in biomolecular systems, while exhibiting properties akin to low-dimensional models, either due to their inherent symmetry or the rigidity of the molecular assembly. The ultimate goal is to explore such properties and to dissect intra- and intermolecular dispersion interactions from other factors such as stronger non-covalent attractions, steric and solvation effects. Theoretical studies will be carried out on the synthesised supramolecular systems and/or models thereof. The objectives are two-fold. On one hand, we wish to distinguish dispersion forces in host-guest interactions and better understand the interplay with solvent exclusions effects. On the other hand, to benchmark and refine procedures to analyse interaction energies, irrespective of molecular size. In the first funding period, we have successfully identified a group of supramolecular structures with great promise for the study of comparative host stability, guest affinity and solvent effects. Theoretical tools for the quantification of dispersion have also been developed and tested in cooperation with other research units in the Priority Programme. In the second phase, we will expand the scope of our project to the study of reactivity inside supramolecular hosts and the impact of dispersion forces in intermediates and reaction products.

 

Bretislav Friedrich, Berlin and Alkwin Slenczka, Regensburg

 

Configurations of van der Waals complexes controlled via London dispersion forces as revealed by means of Stark spectroscopy in
He-nanodroplets

 

By means of high resolution Stark spectroscopy on electronic transitions molecular van der Waals complexes are studies which. The formation of such complexes inside helium nanodorplets is determined by long rage London dispersion forces. Combined experimental and theoretical investigations head to reveal the influence of long rage London dispersion forces on the configuration of such complexes. In addition to molecular complexes the results obtained so far show an interesting aspect of the project to study microsolvation of molecules in helium droplets. Thereby, the dispersion interaction between non-polar organic molecules and helium atoms plays a dominant role. This is an extreme example for steering by London dispersion forces.

 

Markus Gerhards, Kaiserslautern

 

Dispersion interactions in isolated molecules and molecular aggregates analyzed by IR/UV and Raman/UV double resonance spectroscopy

 

In this project we analyze a variety of molecular systems with competing structural arrangements in which London dispersion forces are important for the energetic preferences. Furthermore, dimers that are only stable if dispersion interactions are strong enough are considered, like substituted hexaphenyl ethane dimers with e.g. unusually short distances between H atoms. Both the influence and quantity of dispersion forces will be investigated with the help of spectroscopic techniques in molecular beam experiments yielding vibrational frequencies and electronic energies in comparison to quantum chemical methods. One set of systems to be investigated are ether–alcohol clusters in which the alcohol molecule can form a hydrogen bond to the ether oxygen or to a pi cloud and where a varying alcohol side-chain is able to control the docking preference due to changed amounts of dispersion interactions. Alternatively, an attached aromatic alcohol molecule can interact with different alkyl side chains, driven by the strength of dispersion interactions. A further class of investigated clusters are aggregates of asymmetric ketones (including protected amino acids) with alcohol molecules. Here, different orientations of the side-chains of the alcohol molecule with respect to the two lone pairs of the carbonyl oxygen atom are possible. All analyses, both for the ether–alcohol and ketone–alcohol clusters, describe a critical balance between nearly isoenergetic structures, i.e. difficult cases for theoretical predictions are presented, since the uncertainty with respect to energetic differences can be in the region of the difference of zero point energies. We focus our molecular beam investigations on the electronic ground state of neutral and partly ionic clusters, but also on the excited state of the neutral species by applying a variety of combined mass- and isomer-selective IR/UV techniques as well as stimulated Raman/UV techniques. Furthermore a new combined IR/Raman/UV variant which is important for isomer-selective measurements will be developed and applied. Our experimental investigations will be performed in close cooperation with other working groups in the field of complementary spectroscopic methods, theory, and synthesis. The synthetic groups also provide us with specifically deuterated or fluorinated compounds. It is a general aim to offer a variety of experimental results in comparison with theoretical predictions. By this an improvement and development of (new) theoretical methods can be achieved in order to get a better quantification of London dispersion, including especially the analysis of electronically excited states.

 

 

Ruth Gschwind, Regensburg

 

London Dispersion in Brønsted Acid Catalysis


 The assessment of dispersion interactions in the transition states of stereoselective catalysis is a very valuable but challenging task, which is so far mainly based on theoretical calculations. For the chiral BINOL-based phosphoric acids used in Brønsted acid catalysis strong contributions of dispersion interactions are expected in their stereodecisive transition states. However to our knowledge an experimental set-up to measure the role of these dispersion interactions is still missing. Recently, we got a detailed insight into the structures and hydrogen bond properties of the binary complexes of chiral phosphoric acids with imines. In addition, we developed the DTS-hn method (decrypting transition states with light) the first experimental access to active transition state combinations and applied it to the transferhydrogenation of imines using chiral phosphoric acids. In this project now the relative dispersion donor energies of various imine/chiral phosphoric catalyst pairs in their ground state structures shall be experimentally measured using dispersion energy balances. Three different experimental dispersion balances and accompanying theoretical calculations are planned to assess contributions from solvent effects, potential changes in the H-bonds or experimental error. The derived dispersion energy values will be compared to the synthetic results of the catalyses and interpreted with the help of the DTS-hn method and theoretical calculations. Overall this combined experimental and theoretical approach is expected to give an assessment of the importance of dispersion interactions in the transition states of Brønsted acid catalysis as well as of their potential changes between ground state and transition state structures.

 

Stefan Grimme, Bonn

 

Modeling of London Dispersion Interactions in Molecular Chemistry

 

The accurate account of London dispersion interactions and their 'chemical' analysis by modern quantum chemical methods are the central themes of this project. Based on our insight and experience with dispersion corrected mean-field (DFT-D/NL) methods we will continue to contribute to gaining a thorough understanding and quantification of London dispersion interactions in molecular systems with projects in the three areas: method development, joint applications with and support of other participants in theoretically difficult or non standard cases. In detail, the following topics will be considered: - automatic generation of conformation ensembles and cluster structures even for large systems by a new, separately developed composite procedure based on a robust and accurate tight-binding quantum chemical method and the new intermolecular force field, - molecular thermochemistry in solution and computation of reaction barriers and mechanisms with an emphasis on dispersion control, - energy decomposition analysis of non-covalent interactions to reveal the importance of dispersion in particular regarding dispersion energy donor (DED) vs. anti-DED (de-stabilising) behavior, - further development of the D4 model including many-body dispersion effects - coupling of the non-local VV10 density functional with excited state quantum chemistry methods and interpretation of experiments involving electronic excitation Particular attention will be paid to the effect of dispersion on the chemical property of interest. With the applied dispersion corrected density functional methods this is technically and conceptually easily possible because the electronic and dispersion energies are assumed to be additive and the correction can easily be switched on/off. In addition the proposed D4 model (in comparison to D3) will allow to investigate the charge dependence of dispersion effects in particular for metallic systems as well as a more accurate consideration of the many-body dispersion energy. The higher accuracy of the new DFT-D4 methods in combination with improved sampling techniques will lead to overall significantly increased reliability of the treatments.

 

Christof Hättig, Bochum

 

Investigation and modelling of dispersion interactions in electronically excited states and the effects of dispersion interactions on electronic excitations


The aim of the project is to achieve a better understanding of the connection between the electronic excitations in a chromophore and its dispersion interaction with an environment. Important research questions that will be addressed are: In which chemical situations and for which kind of electronically excited states is it important to account for the change of the dispersion interaction upon electronic excitation in its quantum chemical description? Are there situations and/or chromophores where the dispersion interaction with the environment changes significantly the excitation energy or the electronic structure of the excited state or the order of the excited states? An additional object of the project is the development of the dispersion contribution to the self-consistent reaction field model PE (polarizable embedding) which accounts for the dispersion interaction during the calculation of the wavefunction parameters for the ground and electronically excited states. A posteriori corrections will thereby be avoided because they lead in general to qualitatively wrong potential energy surfaces in the vicinity of avoided crossings. This is done because we aim at a QM/MM method, which includes the dispersion interaction upon electronic excitation processes and is suitable for the calculation of excited state structures and relaxation processes. This will be implemented at the example of the PERI-ADC(2) and PERI-CC2 methods. For the computation of dispersion coefficients for larger molecules with a correlated wavefunction method we will in addition extend an existing RI-CC2 code for frequency-dependent polarizabilities in ground and excited states for imaginary frequency and for the calculation of Cauchy moments.


Andreas Heßelmann, Erlangen

 

Intra- versus intermolecular forces: the influence of solute-solvent interactions on the structure and properties of extended molecules

 

The geometric and electronic structure of extended molecules can be significantly influenced by a solvent environment when compared with the gas phase. Since, in the vast majority of cases, the study of chemical processes takes place in the liquid phase, the knowledge about the impact of the solute-solvent interactions therefore are crucial for the interpretation and prediction of chemical structures and properties. The application of theoretical methods for this purpose, however, constitutes a highly challenging task, because the solute-solvent system can only be properly described by completely sampling the complex conformational landscape of the system. The aim of this project is to employ a combination of classical molecular dynamics (MD) force field methods and quantum chemistry methods for describing the interactions between a molecule with solvent molecules and to quantify both inter- and intramolecular interactions in the system in order to define structural changes of extended molecules induced by various solvent environments. The MD simulations will be done in order to scan the conformational space of the system, and MD quenching will then be used for extracting a set of structures (molecule plus shell of solvent molecules) which can then be studied further with accurate quantum chemistry methods. For the latter, our recently developed incremental molecular fragmentation scheme will be employed which can be used to decompose the total energy of a molecule into bonded and nonbonded energy contributions. By using symmetry-adapted intermolecular perturbation theory (SAPT) methods for the latter, the interactions in the solute-solvent system can then be characterised by individual interaction energy terms, like electrostatic and dispersion interactions. With this, a comparison between intramolecular interactions of extended dissolved molecules to the interactions in the gas phase can thus give an insight in the dependence of intramolecular dispersion interactions on the outer environment.

 

Petra Imhof and Ana Vila Verde, Berlin

 

Dispersion interactions in fluorinated biopolymers


The structure and function of biopolymers are determined by the balance of different types of interactions: electrostatic, dispersion, and hydrophobic interactions. The latter are important in understanding the “hydrophobic effect” associated with aggregation of apolar solutes in aqueous environment. Dispersion interactions thus play a major role in the biopolymer structure and dynamics, leading to and stabilizing protein assemblies. Fluorination has been shown to modulate the properties of small molecules by altering the balance between electrostatic and dispersion interactions. Similar effects can be expected for peptides, proteins and other biopolymers. To explore the impact of fluorination on hydrated biopolymers, we will employ molecular simulations at atomic level detail, in comparison with Raman experiments provided by our collaboration partners. First principles simulations of fluorinated and non-fluorinated small molecules in gas phase and in solution will allow us to dissect the balance of electrostatic and dispersive interactions on an electronic structure level. These calculations will furthermore serve as reference data for the development of classical force field parameters. Using these parameters, classical molecular dynamics simulations on a larger scale will reveal the impact of fluorination on the dispersion interactions in biopolymers and their dependence on polymer length, conformation, and structural flexibility. These aspects, which are important for proteins, cannot be studied in the very small molecules typically used as analogues of amino acid side chains. However, direct dispersive interactions between hydrophobic groups in solution do depend on the distance of these groups and thus the shape and conformational dynamics of the polymer. Our simulations of selectively fluorinated biopolymers will probe whether dispersion interactions are the dominant contribution to hydrophobic attraction and may thus account for changes in protein structure and structural stability in fluorinated proteins. This understanding can ultimately be used to modify the balance of the different interactions and thereby control protein properties via specific fluor-substitution.

 

 

Georg Jansen and Stephan Schulz, Essen

 

Combined Quantum Chemical and Experimental Study on Metal-Metal Interactions of Heavy Group15 and Group16 Compounds

 

In this project we investigate the role of dispersion interactions in chemical compounds of the atoms Sb, Bi, and Te, combining synthesis of corresponding compounds with quantum chemical calculations and structural characterization via X-ray crystallography. In the first funding period of the project dispersion-dominated non-covalent interactions between these atoms and with pi systems of arenes were found to be decisive for the formation of crystal structures of their compounds. They also significantly contribute to the stabilization of conformers of molecules containing several non-covalently linked heavy group 15/16 metal atoms and may cause, for example bond angles below 90 degree. In the second funding period the influence of electron-donating and -withdrawing ligands of the metal atoms comes into focus. Their effects on the one hand shall be predicted with the help of quantum chemical calculations, where intermolecular perturbation theory with its possibility of quantitative computation of dispersion and further interactions will play an important role. On the other hand we will synthesize and structurally characterize promising candidates with particularly strong interactions. Furthermore we will synthesize homo and hetero bimetallic complexes with rigid aromatic ligands, which allow for a tuning of the distance between the metal atoms. Its consequences on intra- and intermolecular interactions shall be understood through structural characterization and quantum chemical calculation. An improved understanding of the anisotropy of the combination of attractive dispersion and repulsive steric forces shall be achieved through intermolecular perturbation theory calculations of the interactions of chemical compounds containing group 15 and 16 metal atoms with test particles, utilizing the results for qualitative predictions of the structures of molecules and molecular aggregates.

 

Willem M. Klopper, Karlsruhe

 

Computational determination of accurate bond energies of dispersion-dominated systems in the gas phase

 

The primary aim of the project is the accurate determination of ground- and excited-state dissociation energies (D0) and electronic binding energies (De) of dispersion-dominated complexes of (hetero-)aromatic molecules (denoted M) with dispersively bound solvents (denoted S) using computational methods. Examples are complexes of M = carbazole, 1-naphthol with S = Ne−Xe, N2, CO. Accurate experimental data are available from stimulated-emission pumping/resonant two-photon ionization (SEP/R2PI) spectroscopy. Computationally, a combination of (dispersion-corrected) density-functional theory and high-level coupled-cluster methods will be applied, which include explicitly-correlated wave functions in order to converge quickly to the limit of a complete basis-set expansion. The established De and D0 values will serve as benchmarks for the development of novel computational methods including explicitly-correlated symmetry-adapted perturbation theory and Green's function-based methods. It is expected that the combined experimental/theoretical benchmarks as well as the new computational methods will be useful for other projects of the Priority Programme
SPP 1807.

 

Ralf Ludwig and Sergey Verevkin, Rostock

 

Competition between hydrogen bonding and dispersion forces in ionic and molecular liquids by means of spectroscopic and thermodynamic methods

 

The competition between hydrogen bonding and dispersion forces in ionic and molecular liquids will be studied by means of infrared, terahertz, several thermodynamic methods and quantum chemical calculations including dispersion correction. The important role of dispersion forces on the formation of ionic and molecular clusters will be investigated for all phases: the solid, the liquid and the gas phase. The structure of the clusters present in each phase will be determined by X-ray and spectroscopic methods whereas changes during phase transition will be probed by thermodynamic methods. For that purpose, a well selected set of ionic and molecular liquids will be synthesized that allows to control of noncovalent interactions. The special feature of the molecular liquids is that the molecules are mimics of the cations used in the ionic liquids. This way we can study the different role of dispersion forces while changing from ionic to molecular liquids and switching off the strong Coulomb interaction. Moreover, the compounds are designed such that we can increase the dispersion forces in a controlled way while weakening hydrogen bonding at the same time. The combined spectroscopic methods (MIR, FIR, THz), thermodynamic methods (calorimetry, thermogravimetry, vapour pressure measurements) and theoretical approaches (DFT and Gn) allow to study the subtle balance between Coulomb interaction, hydrogen bonding and dispersion forces in these model compounds. In particular the role of dispersion forces can be analyzed in the liquid phase, wherein most practical chemistry occurs.

 

Werner Nau, Bremen

 

London Dispersion Interactions inside Macrocycles

 

We propose to investigate the contribution of London dispersion interactions (LDI) to the molecular recognition process of macrocycles, prominently cucurbit[n|urils (CBn) and to the control of chemical reactions inside their inner cavity. CBn are water-soluble macrocyclic host molecules of the molecular container type, that is, they are able to encapsulate numerous organic guests inside their hydrophobic cavity. The driving force for binding inside the inner cavity of CBn is primarily traced back to a hydrophobic effect, but LDI present an important modulator. Out of the original six experimental lines of investigation, five will be further pursued in the second project phase: 1) Building up on our quantification of LDI in the binding of noble gases (He, Ne, Ar, Kr, and Xe) to the smallest CBn homologue, CB5, we plan to study the potential of different docking cations to modulate LDI in the resulting CB5•noble gas•cation complexes. 2) We will extend our investigation on the high-affinity binding of strongly polarizable borate clusters of the type (B12X12)2– to substituted carboranes, which in contrast to the dianionic borate clusters possess a neutral core. In order to dissect solvent effects from LDI, we will also study the solvent isotope effect for this type of complexes. 3) As an example of supramolecular catalysis, we have investigated the dimerization of cyclopentadiene inside CB7, for which we have found a million-fold rate enhancement; this study will be extended to methylcyclopentadiene to introduce chemoselectivity into the catalytic reaction. 4) We propose to assess the relative abundance of inclusion versus exclusion complexes in the gas phase as an interesting approach to study LDI. Through collaborations, we have started to investigate a series of organic ammonium ions with CB6 in the gas phase. For direct comparison, we already determined the binding constants of the ammonium ions to CB6 in water during the first phase of the priority programme. 5) The inversion process of CBn macrocycles in the gas phase will also be examined, which is thought to be driven by intramolecular LDI. Throughout the project, NMR spectroscopy, isothermal titration calorimetry, dye displacement titrations, organic synthesis and quantum-chemical calculations will be used to determine the binding affinities as well as thermodynamic parameters and, thus, to evaluate the importance of LDI.

 

Expired Project: The Hydrophobe Challenge

 

Jan Paradies, Paderborn

 

Stabilization of encounter complexes of intermolecular frustrated Lewis pairs by dispersion energy donors


Dispersion interactions have now been identified in all fields of chemical research giving rise to unusual physical properties, conformations, selectivity of reactions and stabilization of overcrowded molecules. One of the most fascinating aspects is that steric bulk in the form of dispersion energy donors can also lead to the thermodynamic stabilization of molecules resulting in spectacular bonding situations or in extremely short intermolecular distances. This research project focusses on the exploitation of dispersion energy donors for the stabilization of the encounter complexes of intermolecular frustrated Lewis pairs. The triaryl derivatives of phosphorus, nitrogen and boron derived Lewis bases and Lewis acids will be equipped in 3 and 5 position of the aromatic rings with dispersion energy donors. Rigid alkyl fragments such as methyl, isopropyl and tertbutyl will provide the necessary dispersion energy to stabilize the corresponding encounter complex of the Lewis acid and Lewis base. All in all, 54 combinations of dispersion energy stabilized encounter complexes are available, which will be investigated by nuclear-Overhauser enhanced NMR spectroscopic methods. The directionality of the corresponding intermolecular interactions will be analyzed offering the elaboration of solution structures. Furthermore, the association constants will be determined at different temperatures enabling for the determination of the thermodynamic parameters. Furthermore, the activity of the frustrated Lewis pairs in the hydrogen splitting will be investigated connecting dispersion stabilization of the encounter complexes to catalytic activity. The experimental data is of high value for the quantum mechanical description of dispersion interactions offering the improvement of accuracy of dispersion-corrected density functional theory. Our synthetic investigations will be complemented by quantum-mechanical calculations in strong collaboration with S. Grimme. The structure of each of the Lewis acid and Lewis base will be optimized by DFT-based methods and subsequent energy decomposition analysis will specify the role of the various energy contributions. Ultimately, this provides a qualitative and quantitative analysis of interacting dispersion energy donors on a highly systematic level. In summary, this approach to dispersion energy donor-stabilized encounter complexes of Lewis pairs strongly connects to all of the key-topics of the SPP 1807: - synthesis of novel dispersion energy donor -stabilized structure - quantitative evaluation of dispersion energy donors in connection with intermolecular complexation, - solvent dependency of dispersion energy donor-stabilized structures and - transition-state stabilization through dispersion energy donors

 

Frank Neese and Giovanni Bistoni, Mülheim an der Ruhr

 

Quantifying London dispersion effects through novel local correlation techniques

 

During the first founding period, we developed an energy decomposition scheme within the domain-based local pair natural orbital coupled-cluster (DLPNO-CCSD(T) framework, which allows for a physically sound decomposition of the accurate DLPNO-CCSD(T) energy into additive, chemically meaningful contributions. This method, also called “Local Energy Decomposition” (LED) analysis, can be used for quantifying the elusive London dispersion component of the interaction energy between an arbitrary number of fragments or molecules. Challenging applications of this scheme will be carried out during the second funding period, with the final aim of contributing to the rational control of London dispersion effects on chemical reactivity. These applications will be carried out in collaboration with several experimental groups and include: (i) complex organocatalyzed reactions; (ii) molecular balances for the quantification of London dispersion effects in solution; (iii) the study of the coordination bond in organometallic chemistry. Moreover, further developments of the LED analysis will be carried out during the second funding period. In particular, we will implement in the ORCA code an open-shell version of this scheme that will allow for a tremendous increase on the number of systems that can be studied. In addition, a series of simple yet powerful tools for the spatial analysis of the different components of the LED analysis will be also implemented in ORCA.

 

Melanie Schnell, Hamburg

 

Intra- and intermolecular dispersion forces: Understanding complex formation, aggregation, and the effect of solvation using a bottom-up approach

 

The goal of the research program is to significantly advance the current understanding of the role of dispersion interaction in molecular recognition and intramolecular structures by systematically exploring model systems using high-resolution rotational spectroscopy. We will continue our successful work on spectroscopically investigating and characterizing molecular systems in the gas phase to understand and quantify intra- and intermolecular interactions. In a bottom-up approach, we will study basic processes relevant for understanding molecular aggregation and finally the transition from the gas to the bulk phase, as well as the influence of solvation on dispersion. For the second funding period, we can now build on a solid, fruitful, and creative network of synthesis, spectroscopy, and theoretical chemistry groups from the SPP, as described above. We propose to work on the following aspects: In project 1, it is our goal to further investigate qualitatively and quantitatively the role of dispersion for a variety of molecular complexes. We will continue our work on ether-alcohol complexes (which already let to a number of surprises in the first funding period) and extend it to ketone-alcohol complexes as well as chiral species, in collaboration with our partner groups. We aim at gaining a complete understanding of how dispersion contributions influence the structures of complexes and which role conformational flexibility plays. In a second project, we will study aggregation of molecules, such as those with extended π systems, and we will thus explore the transition from individual molecules to the bulk on the molecular level. The anticipated results on a molecule-by-molecule build-up mechanism will also be of interest for the formation of soot particles as relevant for combustion as well as of dust grains as relevant for astrochemistry. In project 3, we will investigate how dispersion is influenced by solvation. We will study molecular complexes, such as diphenylether-methanol, and investigate how structural preferences upon clusters formation are modulated by stepwise solvation of the complex. The controlled conditions in a molecular jet are ideally suited for such experiments. We aim at understanding how the effect of solvation depends on the different molecule systems, and how this can be predicted and finally controlled.

 

Peter R. Schreiner, Gießen

 

London Dispersion as a design element to control molecular structures and chemical reactivity

 

The goal of the project is to improve our conceptual understanding of London dispersion (LD) interactions. We strive to achieve this using synthetic, spectroscopic, and computational techniques as follows: a) Synthesis and detailed structural characterization of the re-discovered Gomberg-system (i.e., dimers of trityl derivatives) that are only made possible by using LD interactions; b) determination of a relative scale of dispersion energy donors (DEDs) utilizing “molecular balances” based on hydrocarbons only (cyclooctatetraene and bifluorenylidene derivatives); c) examination of the effects of lipophilic groups (i.e., DEDs) in oligopeptide catalysts as well as substrates in organocatalytic transformations (acylation, Dakin-West reaction). Ideally, the concepts lead to improved ways of directing reactivity and selectivity, novel materials, and the improvement of high-quality quantum mechanical methods for increasingly larger systems. A good part of our work is directly connected to projects of other expert groups from theory and spectroscopy.

 

Martin Suhm, Göttingen

 

London vs. Keesom and Debye forces: From FTIR cluster spectroscopy of ambivalent alcohol complexes towards intermolecular energy balances

 

London dispersion forces can tip the balance between different hydrogen bond docking sites of an alcohol molecule to a multifunctional acceptor molecule. By selecting systems with low barriers between energetically nearly degenerate docking sites and by working in a supersonic jet environment, very subtle energy differences can be detected and quantified with linear infrared spectroscopy. By studying a large number of systems with varying dispersion anchors, fortuitous error cancellation can be ruled out and subtle deficiencies of quantum chemical methods in providing balanced descriptions of all intermolecular forces can be uncovered in a systematic way. By choosing chemically similar docking sites, distorting effects from anharmonic zero point energy can be minimized. By including chiral donor and acceptor molecules, chirality recognition effects mediated by dispersion forces can be studied as well. The project concentrates on carbonyl lone pair and alkene π bond face choices, along with the oxygen/π competition, which was in the focus of the first funding period. Improved nozzle and sample preparation designs will be explored. Intense experimental cooperation with UV/IR and microwave experts in the priority programme is planned. Besides testing quantum chemical predictions for intermolecular interactions, such theoretical methods will be used to separate and visualize the London dispersion contribution of substituents in cooperation with theory groups. In the end, the new concept of intermolecular energy balances to probe London dispersion interactions in the gas phase at low temperature will complement the popular intramolecular torsional balances in solution by providing energy-focused information free of bulk solvent influence.

 

Hermann Wegner, Gießen

 

Investigation of London Dispersion Interactions with Azobenzene Switches

 

The effect of bulky groups has been mainly considered according to their repulsive interactions. Recently, it has been realized, that the attractive part of the van-der-Waals interactions, London dispersion, can be utilized as stabilizing element in molecular and reaction design. Despite the progress made, the nature of these forces are still not fully understood. Especially the experimental evaluation of the strength in terms of dispersion of different groups (dispersion donor groups = DDGs) has only been scarcely addressed. A long disputed issue is the effect of solvation on dispersion. In the past years, we could show that the thermal back reaction of azobenzene from the Z- to the E-isomer is ideally suited to address these questions. It allows positioning two DDGs in close proximity. By examining the isomerization rate, the stability of various DDGs can be assayed based on the Bell-Evans-Polyani relationship. Herein, various DDGs ranging from alkyl to structures relevant in life science, such as peptides, nucleobases, etc. will compared according to their dispersion strength. Furthermore, the influence of different solvents on the dispersion strength of given DDGs will be investigated providing essential guidelines, how to design ideal conditions to make the best use of dispersive interactions. The studies will be completed with collaborative efforts with other groups within the SPP 1807 in the area of catalysis, supramolecular chemistry and structural analysis.